Breast cancer remains a significant threat to women's health. X-ray mammograms have long been a standard in screening patients for breast cancer or other abnormalities and also are widely used in diagnosis and treatment planning. X-ray mammography typically records the breast image on x-ray film but more recently digital x-ray image receptors have come into use, as in the Selenia™ mammography system available from Hologic Inc. of Bedford, Mass. and its division Lorad of Danbury, Conn. For mammograms, a cone-shaped or pyramid-shaped x-ray beam passes through the compressed breast and forms a two-dimensional projection image. Any one of a number of orientations can be used, such as cranial-caudal (CC) or MLO (mediolateral-oblique) orientation. More recently, breast x-ray tomosynthesis has been proposed. The technology typically involves taking two-dimensional (2D) projection images of the immobilized breast at each of a number of angles of the x-ray beam relative to the breast and processing the resulting x-ray measurements to reconstruct images of breast slices that typically are in planes transverse to the x-ray beam axis, such as parallel to the image plane of a mammogram of the same breast. The range of angles is substantially less than in computerized tomography, i.e. substantially less than 180°, e.g. ±15°. Tomosynthesis technology is described in U.S. patent application Ser. No. 10/723,486 filed Nov. 26, 2003; a prototype of a unit with at least some of the described features was shown at the 2003 Radiological Society of North America meeting in Chicago, Ill. Additional prototypes are in clinical testing in this country as of the filing of this patent specification. Other approaches to tomosynthesis also have been proposed: see, e.g., U.S. Pat. Nos. 4,496,557, 5,051,904, 5,359,637, 6,289.235, and 6,647,092, published U.S. Patent Applications Nos. 2001/0038861, 2004/066882, 2004/0066884, and 2004/0066904, and Digital Clinical Reports, Tomosynthesis (GE Brochure 98-5493, November/1998). How to reconstruct tomosynthesis images is discussed in DG Grant, “Tomosynthesis: a three-dimensional imaging technique”, IEEE Trans. Biomed. Engineering, Vol BME-19, #1, (January 1972), pp 20-28. The patents, applications, brochures, and article cited above are hereby incorporated by reference in this patent specification as though fully set forth herein.
In clinical use, it can be desirable for a number of reasons to assess both tomosynthesis images and conventional mammograms of the patient's breasts. For example, the decades of conventional mammograms have enabled medical professionals to develop valuable interpretation expertise. Mammograms may offer good visualization of microcalcifications, and can offer higher spatial resolution compared with tomosynthesis. Tomosynthesis images may have different desirable characteristics—e.g., they may offer better visualization of structures that can be obscured by overlying or underlying tissue in a conventional mammogram. However, the inventors named herein have recognized that a challenge arises in assessing tomosynthesis images, either alone or in conjunction with mammograms of the same breast. Tomosynthesis images tend to look different from mammograms in that a given tomosynthesis image may not show anatomical structure seen in a mammogram or in another tomosynthesis image of the same breast; and, to the extent a tomosynthesis image shows structure that also is seen in the mammogram or in another tomosynthesis image, that structure may be at different relative places in the images. This can make it difficult to apply to tomosynthesis images the expertise built over years of experience reading mammograms, and difficult to visualize and assess the same structure from the different types of images. For these and other reasons, the inventors believe that a need exists for further improvements in the generation and presentation of such images to make them more useful to health professionals.